Calibration Device

The present specification relates to systems, methods, apparatuses, devices, articles of manufacture and instructions for calibration of semiconductor structures and sensors, such as MEMS (microelectromechanical system) devices.

According to an example embodiment, a calibration device, comprising: a substrate; a torsional element; wherein a first end of the torsional element is coupled to the substrate; wherein a second end of the torsional element is moveable and defines a capacitance with the substrate; a bias electrode coupled to the substrate; wherein the torsional element and the bias electrode are positioned such that a bias voltage applied between the substrate and the bias electrode creates an electrostatic force between the bias electrode and the torsional element; wherein the electrostatic force causes movement between the torsional element and the substrate; and wherein the movement varies the capacitance.

In another example embodiment, the torsional element is a spring.

In another example embodiment, the second end of the torsional element is coupled to a moveable proof mass.

In another example embodiment, the bias electrode is a first bias electrode positioned apart from and to a first side of the torsional element; further comprising a second bias electrode coupled to the substrate and positioned apart from and to a second side of the torsional element; wherein the first side and the second side are on opposite sides of the torsional element.

In another example embodiment, the bias voltage applied between the substrate and the first bias electrode creates a first electrostatic force that moves the torsional element in a first direction and sets the capacitance to a first capacitance; and the bias voltage applied between the substrate and the second bias electrode creates a second electrostatic force that moves the torsional element in a second direction and sets the capacitance to a second capacitance.

In another example embodiment, the substrate and the torsional element are formed in parallel x-y-planes; and the bias voltage causes the torsional element movement in a z-axis that is perpendicular to the x-y-planes.

In another example embodiment, the torsional element has a trapezoidal cross-section along the z-axis.

In another example embodiment, the electrostatic force simulates a lateral acceleration of the torsional element.

In another example embodiment, further comprising a first bottom electrode and a second bottom electrode, both coupled to the substrate; wherein the electrostatic force moves the torsional element away from the first bottom electrode and closer to the second bottom electrode.

In another example embodiment, the capacitance variation defines a cross-axis sensitivity of the torsional element.

In another example embodiment, further comprising a controller; wherein the controller is configured to calculate the cross-axis sensitivity based on the capacitance variation in response to the bias voltage.

In another example embodiment, the bias electrode is included in a first set of bias electrodes all coupled to the substrate and positioned along both the first side and the second side of the torsional element; further comprising a second set of bias electrodes all coupled to the substrate and positioned along both the first side and the second side of the torsional element.

In another example embodiment, the controller is configured to apply the bias voltage to the first set of bias electrodes and apply a ground voltage to the second set of bias electrodes at a same time.

In another example embodiment, increasing a number of bias electrodes in either or both the first and second sets of bias electrodes increases the capacitance variation in response to the bias voltage.

In another example embodiment, the calibration device is configured to calibrate a fabricated semiconductor structure.

In another example embodiment, the calibration device is configured to calibrate a MEMS (microelectromechanical system) device.

In another example embodiment, the calibration device is configured to calibrate a pressure sensor.

In another example embodiment, the calibration device is configured to calibrate an accelerometer.

In another example embodiment, the calibration device is a metrology structure embedded in a wafer separate from a set of devices to be calibrated.

According to an example embodiment, a method of calculating cross-axis sensitivity using a calibration device: wherein the calibration device includes, a substrate; a torsional element; wherein a first end of the torsional element is coupled to the substrate; wherein a second end of the torsional element is moveable and defines a capacitance with the substrate; a bias electrode coupled to the substrate; wherein the torsional element and the bias electrode are positioned such that a bias voltage applied between the substrate and the bias electrode creates an electrostatic force between the bias electrode and the torsional element; wherein the electrostatic force causes movement between the torsional element and the substrate; and wherein the movement varies the capacitance; and wherein the method of calculating cross-axis sensitivity includes, applying the bias voltage across the bias electrode and the torsional element; measuring the capacitance variation between the torsional element and the substrate; and calculating the cross-axis sensitivity based on the capacitance variation in response to the applied bias voltage.

The above discussion is not intended to represent every example embodiment or every implementation within the scope of the current or future Claim sets. The Figures and Detailed Description that follow also exemplify various example embodiments.

Various example embodiments may be more completely understood in consideration of the following Detailed Description in connection with the accompanying Drawings.

While the disclosure is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that other embodiments, beyond the particular embodiments described, are possible as well. All modifications, equivalents, and alternative embodiments falling within the spirit and scope of the appended claims are covered as well.

1 1 1 FIGS.A,B,C 100 100 100 104 106 108 110 represent an example MEMS (microelectromechanical system) device. The example MEMS deviceis fabricated in an x-y-plane over a semiconductor substrate (not shown), and in some examples may be embedded in a semiconductor wafer having other MEMS devices (not shown). The example MEMS deviceincludes a proof mass, a first springhaving a TOP surface and a BOTTOM surface along a z-axis arising out of the x-y-plane, a second springalso having a TOP surface and a BOTTOM surface along a z-axis arising out of the x-y-plane, and an anchor.

110 106 108 110 106 108 104 104 104 110 110 The anchor(e.g. pivot point) is physically coupled to one end of both the first springand the second spring. The anchoris also physically coupled to the substrate (not shown). Another end of both the first springand the second springis physically coupled to the proof mass. The proof massis sensitive to various forces (e.g. pressure, acceleration, etc.) that cause the proof massto wobble about the anchor. The anchordoes not move since it is physically fixed to the substrate.

1 104 112 2 104 114 104 110 1 2 1 2 A first capacitor (Z) is formed between the proof massand a first bottom electrodethat is attached to the substrate. Additionally a second capacitor (Z) is formed between the proof massand a second bottom electrodethat is also attached to the substrate. As the proof masswobbles about the anchorin response to various forces, capacitance values of the first capacitor (Z) and the second capacitor (Z) also vary. A controller (not shown) calculates values for these various forces (e.g. pressure, acceleration, etc.) based on the capacitance variations Z, Z.

106 108 1 FIG.B Given that semiconductor fabrication is imperfect, the first springand/or the second springmay have an undesired tapering in the z-axis that narrows the further out along the z-axis away from the substrate (i.e. a trapezoidal cross-section as shown in). The taper may not be in the x-y-plane.

106 108 Cross-axis sensitivity is caused by this tapering of the springs,. Cross-axis sensitivity is a torsional, twisting where forces in the x-y-plane cause undesired movement along the z-axis. Cross-axis sensitivity in many microelectromechanical system (MEMS) devices is a critical performance parameter.

For example, vertical (z-axis) MEMS accelerometers are required to be highly insensitive to cross-axis acceleration (accelerations originating in the x-axis and/or y-axis). Even a very small amount of tapering can cause an out-of-spec cross-axis sensitivity problem (e.g. a 0.35 degree taper is enough to cause out of spec performance).

106 108 100 Detecting such very small tapering in the springs,or many other semiconductor structures is very difficult if not impossible during back-end wafer testing often using test probes placed in temporary contact with pads on the MEMS device.

Note, semiconductor structure tapering is different than skew. Skew has a parallelogram cross-section while tapering has a trapezoidal cross-section. Skew may be an important constraint for some semiconductor devices, but may not be an important constraint for other types of semiconductor devices (e.g. skew can present significant problems for MEMS gyro quadrature, but does not cause any issue with Z-accel cross-axis sensitivity.)

Now discussed is a calibration device (e.g. a metrology (i.e. test) structure, a circuit, etc.) for accurately quantifying cross axis-sensitivity due to tapering in semiconductor structures. Such structures in some example embodiments may be part of an electrical circuit or an electrical device. However, in other example embodiments, such structures may be part of an optical device, a meta-material device, an antenna device, or other structures.

In some example embodiments, the calibration device is only used during back-end semiconductor wafer quality sorting (i.e. acceptance criteria for detecting good and bad wafer structures). However, in other example embodiments the calibration device can be used for real-time testing and/or calibration of various semiconductor structures in a chip package and perhaps in operation for functional safety testing over a lifetime of the structure.

Thus in some example embodiments, a set of MEMS devices fabricated on a wafer may each have their own calibration device to measure cross axis-sensitivity. While in other example embodiments, a single calibration device can be part of a separate test structure embedded in the wafer and using back-end test probes is used to characterize (e.g. pass/fail) all of the structures/circuits on the wafer, but which is then later discarded (e.g. diced off).

The calibration device can thus enable closed loop monitoring of critical performance parameters after wafer fabrication. The calibration device can improve process control and reduce a risk of a large amount of fabrication resources being affected by a process excursion before detecting the issue at final test.

2 2 FIGS.A,B 200 200 202 200 204 206 208 210 represent an exampleof a calibration device. The example calibration deviceis fabricated in an x-y-plane over a semiconductor substrate, and in some examples may be embedded in a semiconductor wafer having other circuits, structures, devices, etc. (not shown). The example calibration deviceincludes a proof mass, a first spring(e.g. a first torsional element), a second spring(e.g. a second torsional element), and an anchor.

210 206 208 210 202 206 208 204 204 204 210 210 202 The anchor(e.g. pivot point) is physically coupled to one end of both the first springand the second spring. The anchoris also physically coupled to the substrate. Another end of both the first springand the second springis physically coupled to the proof mass. The proof massis sensitive to various forces (e.g. pressure, acceleration, etc.) that cause the proof massto move (e.g. rotate, twist, etc.) about the anchor. The anchordoes not move since it is physically fixed to the substrate.

1 204 212 2 204 214 204 210 1 2 A first capacitor (Z) is formed between the proof massand a first bottom electrodethat is attached to the substrate. Additionally a second capacitor (Z) is formed between the proof massand a second bottom electrodethat is also attached to the substrate. As the proof massmoves (e.g. rotates, twists, etc.) about the anchorin response to various forces, capacitance values of the first capacitor (Z) and the second capacitor (Z) also vary.

1 2 200 200 A controller (not shown) calculates values for these various forces (e.g. pressure, acceleration, etc.) based on the capacitance variations Z, Z. In some example embodiments, the controller is included in a test probe (not shown) that is brought into contact with a set of test pads on the calibration deviceduring testing. In other example embodiments, the controller is embedded along with the calibration devicein a circuit to be tested.

200 216 218 216 218 216 218 206 208 The calibration devicealso includes a first set of bias electrodesand a second set of bias electrodes. In many example embodiments, all of the first set of bias electrodesare electrically coupled together and all of the second set of bias electrodesare electrically coupled together. These sets of bias electrodes,are used to calculate cross-axis sensitivity for one or more devices that have springs,or similar structures, as will be further discussed below.

216 218 202 204 210 200 216 218 216 218 216 218 2 FIG.A The bias electrodes,are attached to the substrateand do not move; however, the proof massmoves about the anchorin response to either forces during normal operation of the calibration device, or in response to voltages applied to the bias electrodes,which create electrostatic forces during testing and calibration. Also, whileshows the bias electrodes,as flag-like structures, in alternate example embodiments the bias electrodes,can be of any shape (e.g. a rectangle, square, circle, etc.).

216 218 202 212 214 1 2 206 208 In some example embodiments, the first set of bias electrodesare also coupled to a first test pad, and the second set of bias electrodesare also coupled to a second test pad so that they can be accessed by a test probe, perhaps during back-end wafer testing and sorting based on cross-axis sensitivity. The test probe would also be in contact with additional test pads (e.g. to the substrateand the bottom electrodes,) to measure the Zand Zcapacitances. In these example embodiments, the cross-axis sensitivity can be used as a wafer pass/fail criteria for all devices on the wafer that have springs,or similar fabricated structures.

216 218 206 208 200 202 212 214 1 2 In other example embodiments, the first set of bias electrodesare also coupled to the controller, and the second set of bias electrodesare also coupled to the controller so that the controller can calculate cross-axis sensitivity for the first springand the second springusing the calibration device. The controller would also be coupled to additional test pads (e.g. to the substrateand the bottom electrodes,) to measure the Zand Zcapacitances. In these example embodiments, the cross-axis sensitivity measurements can be stored in a memory to be used later during device operation for functional safety or ongoing calibration purposes.

200 200 216 210 204 216 204 206 208 1 2 204 212 214 218 210 1 2 204 212 214 216 218 210 1 2 204 212 214 In some example embodiments, a cross-axis sensitivity of the calibration deviceis tested and calculated as follows. First, when the calibration deviceis otherwise in a quiescent state (i.e. not subject to physical/movement forces), the controller applies an electrical voltage (Vbias) across the first bias electrodeand the anchor. This Vbias voltage creates electrostatic forces between the proof massand the first bias electrodethat simulate a lateral physical acceleration on the proof mass(i.e. the springs,move, rotate, twist, etc.). Second, the controller measures the Zand Zcapacitances between the proof massand the bottom electrodes,. Third, the controller applies the same electrical voltage (Vbias) across the second bias electrodeand the anchor. Fourth, the controller again measures the Zand Zcapacitances between the proof massand the bottom electrodes,. Fifth, the controller grounds both the bias electrodes,and the anchor. Sixth, the controller measures the Zand Zcapacitances between the proof massand the bottom electrodes,.

1 2 216 218 The Zand Zcapacitors change their capacitance based on an amount of tilt/lateral acceleration caused by these three different voltage applications on the bias electrodes,.

216 218 1 2 202 210 212 214 216 218 1 2 In some example embodiments, the controller grounds the first set of bias electrodeswhen Vbias is applied to the second set of bias electrodes, and vice versa, to reduce noise during the capacitance Z, Zmeasurements. Note electrical insulation (e.g. oxide layers) are added between the substrate, the anchor, the first bottom electrode, the second bottom electrode, the first set of bias electrodes, and the second set of bias electrodesas needed to prevent short circuits as various voltages are applied and capacitance Z, Zmeasurements are taken.

2 FIG.B 204 202 206 208 204 202 An exaggerated z-axis tilt in response to the applied Vbias voltages is shown in. As mentioned earlier, this tilting is undesired and results from cross-axis sensitivity. If there was no cross-axis sensitivity, then the proof masswould just move in the x-y-plane of the substrateand there would be no tilt. Thus cross-axis sensitivity caused by tapering in the springs,tilts the proof massin the z-axis perpendicular to the x-y-plane of the substrate.

1 2 216 218 200 With these three sets of Zand Zcapacitance measurements under these three different voltage applications to the bias electrodes,, the controller can calculate the cross-axis sensitivity of the calibration device.

1 2 1 2 1 1 1 2 An example equation for calculating cross-axis sensitivity based on these capacitance Z, Zmeasurements is: Sx=C*(ΔZ−ΔZ), where Sx is the cross-axis sensitivity, C is a proportionality constant dependent upon design of structure and voltage level applied during test, and ΔZand ΔZis the change in Zand Zcapacitance observed when applying test bias.

2 FIG.A 216 218 206 208 216 206 208 218 206 208 Note, whileshows bias electrodes,on both sides of the springs,, in other example embodiments, the first set of bias electrodescan just be on one side of the springs,, and the second set of bias electrodescan just be on the other side of the springs,.

1 2 Also in other example embodiments additional rows of bias electrodes can be added to either or both sides of the torsion springs to further increase a sensitivity of the Zand Zcapacitances measured and thus a more accurate cross-axis sensitivity calculation.

3 3 3 FIGS.A,B,C 300 200 represent a potential non-idealityin the calibration device. For example, in some applications applying the Vbias voltages may introduce an additional non-ideality.

206 208 206 208 204 206 208 3 FIG.B During normal operation of the springs,, as shown in, the physical forces on the springs,from the proof massmovement is substantially equal from top to bottom of the springs,.

3 FIG.C 206 208 216 218 206 208 216 218 206 208 However, as shown in, the electrostatic forces caused by the Vbias voltage may be much larger on the bottom of the proof mass than the top, since the same non-ideal etch process that causes tapering of the springs,also affects the bias electrodes,. This results in a “reverse tapering” of the gap between the springs,and the bias electrodes,over which the electrostatic forces are applied. This effect causes a moment opposite to the moment caused by the tapering in the springs,.

4 4 4 FIGS.A,B,C 400 400 1 2 402 404 300 1 2 402 216 218 404 represent example simulation resultsfor various example embodiments of the calibration device. The simulation resultscompare the Z, Zcapacitancemeasurement to cross-axis sensitivityeven in a presence of the potential non-ideality. The simulation results illustrate a good correlation between the Z, Zcapacitance measurementsby the controller in response to the Vbias voltages on the bias electrodes,and the actual cross-axis sensitivity.

Various instructions and/or operational steps discussed in the above Figures can be executed in any order, unless a specific order is explicitly stated. Also, those skilled in the art will recognize that while some example sets of instructions/steps have been discussed, the material in this specification can be combined in a variety of ways to yield other examples as well, and are to be understood within a context provided by this detailed description.

In some example embodiments these instructions/steps are implemented as functional and software instructions. In other embodiments, the instructions can be implemented either using logic gates, application specific chips, firmware, as well as other hardware forms.

When the instructions are embodied as a set of executable instructions in a non-transitory computer-readable or computer-usable media which are effected on a computer or machine programmed with and controlled by said executable instructions. Said instructions are loaded for execution on a processor (such as one or more CPUs). Said processor includes microprocessors, microcontrollers, processor modules or subsystems (including one or more microprocessors or microcontrollers), or other control or computing devices. A processor can refer to a single component or to plural components. Said computer-readable or computer-usable storage medium or media is (are) considered to be part of an article (or article of manufacture). An article or article of manufacture can refer to any manufactured single component or multiple components. The non-transitory machine or computer-usable media or mediums as defined herein excludes signals, but such media or mediums may be capable of receiving and processing information from signals and/or other transitory mediums.

It will be readily understood that the components of the embodiments as generally described herein and illustrated in the appended figures could be arranged and designed in a wide variety of different configurations. Thus, the detailed description of various embodiments, as represented in the figures, is not intended to limit the scope of the present disclosure, but is merely representative of various embodiments. While the various aspects of the embodiments are presented in drawings, the drawings are not necessarily drawn to scale unless specifically indicated.

The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by this detailed description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.

Reference throughout this specification to features, advantages, or similar language does not imply that all of the features and advantages that may be realized with the present invention should be or are in any single embodiment of the invention. Rather, language referring to the features and advantages is understood to mean that a specific feature, advantage, or characteristic described in connection with an embodiment is included in at least one embodiment of the present invention. Thus, discussions of the features and advantages, and similar language, throughout this specification may, but do not necessarily, refer to the same embodiment.

Furthermore, the described features, advantages, and characteristics of the invention may be combined in any suitable manner in one or more embodiments. One skilled in the relevant art will recognize, in light of the description herein, that the invention can be practiced without one or more of the specific features or advantages of a particular embodiment. In other instances, additional features and advantages may be recognized in certain embodiments that may not be present in all embodiments of the invention.

Reference throughout this specification to “one embodiment,” “an embodiment,” or similar language means that a particular feature, structure, or characteristic described in connection with the indicated embodiment is included in at least one embodiment of the present invention. Thus, the phrases “in one embodiment,” “in an embodiment,” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment.